1. Field of the Invention
This invention relates to improvements in a displacement detecting apparatus, and more particularly to a displacement detecting apparatus for detecting a displacement between an object lens and a measured surface in a non-contact manner by using an optical astigmatism method.
2. Description of the Prior Art
A variety of methods such as an optical astigmatism method, a critical angle detecting method and a knife-edge method have been proposed and put into practical use to measure the displacement between an object lens and a measured surface, the shape of an object, and the surface roughness of the object. Such methods are accomplished, for example, with a non-contact type surface roughness tester which is arranged to measure a displacement in accordance with an absolute level of a focus error signal, and a focus-adjusting type non-contact displacement detecting apparatus which is provided at its object lens with a driving system as a mover and arranged to cause the object lens to follow the measured surface under feedback-control of a focus error signal, and then to detect a displacement between the object lens and the measured surface in accordance with a signal from the driving system as the mover.
The above-discussed displacement detecting apparatuses are adapted to measure the surface roughness or a fine contour of the measured surface. During a measurement operation of such apparatuses, the upper limit of a measuring speed for the measured surface is decided by a frequency response of the mover of the apparatus, and therefore raising the resonance frequency of the mover is required. In order to raise the resonance frequency of the mover, it is effective to minimize the longitudinal dimension of the mover, to shape the mover so as to have a high rigidity, to form the mover of a material having a high modulus of longitudinal elasticity and to raise the rigidity of a support member for the mover.
Furthermore, the above-discussed displacement detecting apparatuses are used to measure the fine deformation and surface roughness of a base plate or the like, and therefore they are required to be small in size. In this regard, the mover for moving the object lens during detection of the displacement is not always disposed symmetrically relative to a moving direction of the object lens.
Difficulties have been encountered in the above-discussed conventional displacement detecting apparatuses, in which the center of gravity of the mover, the center of driving force and the support center of the mover do not coincide with each other in a plane perpendicular to the displacement direction of the mover, thereby generating a moment around a support point for the mover. As a result, a lateral vibration is generated in the mover relative to the longitudinal direction of the mover, thereby lowering the resonance frequency of the mover. This will lower the maximum measuring speed of the displacement detecting apparatus to the measured surface.